1. Field of the Invention
The present invention relates to an amorphous silicon germanium thin film and a photovoltaic element incorporating the amorphous silicon germanium thin film.
2. Description of Related Art
Amorphous semiconductor thin films, such as amorphous silicon films, have been conventionally employed as materials permitting easy and inexpensive fabrication of wide-area semiconductor elements. One well-known technique to form the amorphous semiconductor thin films is a plasma CVD technique. Also, one well-known semiconductor element incorporating the amorphous semiconductor thin film is a photovoltaic element which converts optical energy to electrical energy.
The photovoltaic element is desired to efficiently convert optical energy to electrical energy, and various studies have been directed to improving its energy conversion efficiency. One known technical measure to successfully improve such an energy conversion efficiency is a stacked solar cell. The stacked solar cell has a multilayer structure consisting of a plurality of power generating unit structures in an attempt to take effective advantage of a solar spectrum, and the semiconductor materials employed for respective photovoltaic unit structures are assembled in such a manner to differentiate their respective optical band gaps. For example, amorphous silicon or amorphous silicon carbide having a relatively large optical band gap is employed to constitute the photovoltaic unit structures located closer to a light incidence plane, and amorphous silicon germanium having a smaller optical gap relative to amorphous silicon is employed to constitute the photovoltaic unit structures located remoter from the light incidence plane. The stacked solar cell thus assembled can rely on the amorphous silicon germanium photovoltaic layer to absorb lights at longer wavelengths covering an infrared region which are difficult for amorphous silicon to absorb. This enables the stacked solar cell to efficiently absorb the solar light for power generation.
It is generally known that when desired to form the silicon germanium thin film with fine film properties by a plasma CVD technique, a substrate temperature must be elevated during film formation to reduce the hydrogen content in the resulting film to a certain extent (Jpn. J. Appl. Phys., Vol. 34 (1995), pp. 1741-1747). However, in fabricating the stacked solar cell on a translucent substrate, e.g. a glass substrate, the photovoltaic unit structure comprised of the amorphous silicon germanium layer must be formed on the photovoltaic unit structure comprised of the amorphous silicon layer. In such a case, the substrate temperature elevated during formation of the amorphous silicon germanium thin film acts to subject the underlying amorphous silicon layer to a problematic heat damage. Specifically, the substrate temperature elevated during formation of the amorphous silicon germanium thin film allows a dopant, e.g. boron in a p-type amorphous silicon layer within the previously formed, underlying photovoltaic unit structure, to diffuse into an i-type amorphous silicon layer, or allows a dopant in a transparent electrode layer to diffuse into the p-type amorphous silicon layer. This leads to the reduction in performance of the solar cell layer comprised of the amorphous silicon layer. As a result, it becomes difficult to fabricate high-quality stacked solar cells.
On the other hand, as the substrate temperature during formation of the amorphous silicon germanium thin film is lowered in an attempt to reduce the potential heat damage given to the underlayer in the stacked solar cell, the decreased film property of amorphous silicon germanium results, and again makes it difficult to fabricate the stacked solar cells of high quality.